Sales per square foot — a popular metric used when comparing the profitability of retail real estate — have tumbled in recent years as the retail industry struggles to draw in shoppers.

Those sales, as measured by CoStar, at most public retailers have declined to an average of around $325 per square foot, down from roughly $375 in the early 2000's, the commercial real-estate research firm said in a report.

To be sure, there are still a few bright spots in the industry. Top retailers are managing to grow sales in the brick-and-mortar business, even as more and more shoppers ring up purchases online.

Meantime, mall, shopping center and street-level retail landlords — names like Simon, GGP, Macerich and Taubman — have been looking for ways to keep their properties fresh, innovative and enticing to consumers. Their best interests are aligned with retailers in that they also want to claim the most sales per square foot.

Here are the retailers that are bringing in the most dollars per square foot of real estate, according to CoStar.

5. Lululemon

Pedestrians pass by a Lululemon store.

Scott Mlyn | CNBC

The apparel sector has one clear winner, according to CoStar, and this company falls within the "hot" athleisure category: Lululemon. The retailer does sales of $1,560 per square foot — less than its technology and jewelry counterparts. But apparel is notably a tough sector to draw shoppers toward today.

4. Tiffany & Co.

Jewelry is displayed in the window of a Tiffany & Co. store on Fifth Avenue in New York.

Victor J. Blue | Bloomberg | Getty Images

The jewelry industry is led by Tiffany & Co., which also boasts an impressive presence globally. Drawing shoppers in to have a look at its diamond-studded hardware, Tiffany does sales of $2,951 per square foot.

3. Murphy USA

Source: Murphy USA

Don't forget the gasoline retailers. Murphy USA sells both fuel and convenience merchandise to consumers across America, making it a top contender for dollars within the industry. With about 1,400 retail stores, Murphy USA does $3,721 in sales per square foot.

2. Generation Next Franchise Brands

Source: Reis & Irvy’s | Vimeo

Chances are you haven't head of Generation Next Franchise Brands. But this retailer is present in movie theaters, malls, airports, hospitals, tourist attractions, and more across America with its frozen yogurt vending kiosks. Leading the food-service retail space, Generation Next Franchise does sales of $3,970 per square foot.

1. Apple

Shoppers and salespeople at an Apple store in New York.

Scott Mlyn | CNBC

You could've guessed it. The top retailer, based on sales per square foot, is Apple, CoStar found. This might not come as a surprise for many, considering the hefty price tags on many of the technology company's products, which range from MacBooks, to iPad devices to the Apple Watch. Apple does $5,546 in sales per square foot.